Shuttle valves are well known in the industry. See for example U.S. Pat. No. 4,253,481 which discloses a cushioned shuttle valve. Shuttle valves generally have two inlets and one outlet. The shuttle valve member inside the valve body moves from a position allowing fluid to flow from a first inlet to the outlet when the pressure in the first inlet is higher than the pressure in a second inlet. When the pressure in the second inlet exceeds the pressure in the flow passage now defined by the first inlet and the outlet, the shuttle will move from its position closing the second inlet to a position closing the first inlet and opening the second inlet thereby changing the flow path. The fluid may also flow in a reverse direction from the outlet to the inlets.
In some applications, particularly those in sub-sea oil production, the ambient pressures operating on the valve may be extremely high in excess of 2,000 psi. Certain of the shuttle valves used in such applications were found to have the shuttle oscillate between the valve seats causing chatter during opening movement. The frequency of oscillation in high ambient pressure conditions could be as high as 20 oscillations per second. Pressure oscillation can, if of a high magnitude, cause damage to the valve and other equipment to which it is connected, similar to “water hammer” which can occur in certain piping systems with a sudden change in flow for example the sudden closing of a valve. “Hammer” has been found to be particularly pronounced when high velocity flow rates are used.
Such shuttle valves may be employed in connection with underwater oil field equipment. For example, the lower ends of two hydraulic fluid accumulators (tanks with air space in the top) may be tied to the inlets of a shuttle valve. Each of two high pressure flow lines feeding the accumulators may be connected thereto between the shuttle valve and one of the tanks. The outlet may be connected to a hydraulic cylinder on a blow out preventer. The outlet of the shuttle valve will supply fluid from whichever one of the flow lines is the higher pressure.
It is especially important that underwater shuttle valves have a long trouble-free life because of their inaccessibility and the extreme expenses to replace or repair one. Additionally, the shuttle valve and the hydraulic system should operate to prevent collapse of the flexible hydraulic supply lines. Due to the high working pressures, up to several thousand (e.g. 5,000) psi the possible differential pressure on shuttle valves can be very high. Sometimes a resulting high momentum of a shuttle as it moves uncontrolled from one seat to the other seat creates enough impact to break or crack the cage or cause it to be warped thus disrupting proper valve operation. Thus, slowing down of the shuttle during at least a portion of its opening movement can reduce the final speed of the shuttle as it moves toward a closed position against a valve seat and reduce shuttle oscillation.
Thus, there is a need for an improved shuttle valve which requires few parts in order to effect smooth operation.